The present invention relates to magnetic recording and reproducing with respect to a magnetic disk device, a magnetic tape device, or the like, which is equipped with a composite head for conducting signal recording through an induction head and conducting signal reproducing through a magneto-resistance effect type of magnetic head, and particularly to reproducing data with error relief.
Read error relief in general involves detection of an error during reading of data, and thereafter repeating the reading with and/or without changing various characteristics of the apparatus to prevent further error, until there is relief from the error, that is until the data is read without error. Read error relief has been proposed to counter an unstable phenomenon inherent to an induction magnetic head, such phenomenon being known as the wiggle phenomenon. The wiggle phenomenon is defined as a phenomenon that manifests itself as waves in the reproduced signal wave form, and is thought by the present inventors to be due to an unstable magnetic domain structure that occurs during the recording operation.
In a magnetic disk device for conducting signal recording and reproducing relative to a magnetic disk, an induction magnetic head is commonly used for both signal recording and signal reproducing. For such a system, error relief is provided with a re-try operation executed when a disk controller (hereinafter referred to as "DKC") determines that a read error has occurred; the re-try operation involves again reading the same portion of the disk repeatedly until the read error is not detected or until a predetermined number of re-tries has been completed. Re-try relief also involves an offset operation, wherein the DKC upon detecting a read error will instruct a positioning circuit to change the positioning of the read head relative to the data track in a direction transverse to the data track. That is, upon receipt of the offset operation instruction from the disk controller, the positioning circuit controls a positioning mechanism to repeat the offset operation (an operation of slightly displacing the head inward or outward in a track-width direction) until a read margin is maximum (the read margin is maximum when the reproduced output is maximum and the read error is minimum).
If the read error cannot be relieved even after using the above re-try operation and offset operation, because of the wiggle phenomenon, the following operation would be conducted. The following operation is based upon a recognition that the wiggle phenomenon occurring in the reproduced wave form is dependent upon the magnetic domain structure of the magnetic head at the time when the write-in operation is conducted. The error relief operation then induces a magnetic flux in the magnetic film of the magnetic head by conducting a predetermined write-in operation. The magnetic flux thus induced varies the unstable state of the magnetic domain structure to a stable state.
The above prior art is effectively used to prevent the wiggle phenomenon from occurring when conducting reading with the induction type of magnetic head, which induction type of magnetic head is commonly used for both signal recording and reproducing in the device. However, for a magnetic disk device in which an induction type of magnetic recording head is used for the recording operation and a magneto-resistance effect type of head (MR head) is used for the reproducing operation, the above mentioned prior art read error relief operation is not effective to a sufficient degree, because the signal recording head and the signal reproducing head are different from each other.
The reproducing operation using the magneto-resistance effect type of head will now be described. Ordinarily, the head magnetic film having the magneto-resistance effect (MR film) conducts its magnetization rotation or crystalline phase change smoothly in accordance with the magnetic field occurring from the magnetic orientation found on the data track of the magnetic disk that is adjacent the head, which causes the electrical resistance of the head magnetic film to be varied corresponding to the variation of the magnetic field produced by the relatively moving data track. The variation of the electrical resistance of the head magnetic film as described above is detected as a voltage variation by supplying a DC read current (bias current) to the magnetic film.
In the magneto-resistance effect type of head, it has been known that the head magnetic film has a magnetic domain structure, and thus phenomena called Barkhausen Noise and Base Line Shift occur. Both phenomena are caused by the behavior of the magnetic domain structure of the head magnetic film, and the wave form of the reproduced signal during the read operation is therefore greatly deformed as a result of such phenomena. The Base Line Shift is defined as a phenomenon characterized in the zero level of the reproduced signal wave form (base line) shifting to a positive or negative side, and this Base Line Shift is considered to be induced due to occurrence of hysteresis caused by a discontinuous motion of the magnetic domain. The Barkhausen Noise, in general, is characterized by noise in the reproduced signal caused by the Barkhausen effect, which is the occurrence of minute jumps in the magnetization of a ferromagnetic or soft magnetic substance as the magnetic field is increased or decreased over a continuous range.
Generally, in order to perform a read operation in a magnetic disk device, a signal pulse is formed for each peak of the reproduced signal wave form, and then the signal pulse is passed through a signal discriminating window to detect data on the basis of the presence or absence of the signal pulse within the signal discriminating window. However, with respect to a reproduced signal wave form having Barkhausen Noise or Base Line Shift, the peak position of the signal is deviated along the time axis and deviated in amplitude. Therefore, the signal pulse produced by detecting a peak is deviated along the time axis relative to the time position of the signal discriminating window, and thus a read error occurs. Also, because of the amplitude deviation, a quasi-peak occurs in the reproduced signal wave form to induce plural and sometimes false signal pulses in the signal discriminating window, and thus a read error occurs.
Densification of the magnetic disk device has happened in the past and will be further promoted in the future. This densification will necessarily cause the signal discriminating window to be further narrowed in the future. Therefore, the possibility that the read error occurs due to Barkhausen Noise or Base Line Shift will be increased further in the future.